Pretreatment of fiberous biomass

ABSTRACT

A conventional agricultural “cuber” machine was modified to transform fibrous, low density cellulosic biomass into a mechanically stable form suitable for use as a feed stock to a bulk flow torrefier process without requiring the addition of a “binder” or other such adjuvant. Certain disclosed embodiments of the product concern a compact “cube” or “thin puck” of raw cellulosic biomass having a density of from 4 to 15 times the bulk density of the shredded raw biomass or from 20 to 32 lb/cu ft. The moisture content is below 10%, typically 3-8%. The strength of the product as measured by dropping the product onto a hard surface from a height of 3 ft. will not produce more than 10% breakage. The products of the present invention can be produced having any desirable dimensions, such as substantially square-, rectangular- or parallelogram-shaped product having at least one dimension of from about 5 to about 30 millimeters, which corresponds to the dimension of the die  2  in FIG.  2.  The length of each extrudate is determined by the angle of the deflector plate  1  in FIG.  2.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of International Application No. PCT/US2017/056637, filed Oct. 13, 2017, which was published in English under PCT Article 21(2), which in turn claims the benefit of U.S. Provisional Patent Application No. 62/408,523, filed on Oct. 14, 2016. These prior applications are incorporated herein by reference in their entirety.

FIELD

The present invention concerns a system and process for converting fibrous biomass raw material into a product that facilitates subsequent torrefaction.

BACKGROUND

Torrefaction is a process whereby lignocellulosic materials are heated in an oxygen free environment to temperatures ranging from 240° C. to over 300° C. In this process residual moisture is evaporated and the cellulosic fractions are decomposed to volatile, oxygen-containing compounds having a lower energy of combustion than the more concentrated, carbon-containing torrefied product. This results in a product having a higher energy based on the weight of the biomass.

However, the raw biomass, especially that from highly fibrous sources such as shredded cedar or Empty Fruit Bunches (EFB), which result from the harvest of palm oil, has an extremely low bulk density. As a result, the reactor systems used for torrefaction of these low density fibrous materials must be very large relative to their throughput and the fibrous character makes the bulk flow properties of these fibrous solids difficult to accommodate in the reactor's design. Conventional “pellet” mills, which produce dense pellets from these fibrous raw materials, must expend a great deal of energy to mill the material fine enough and make it flowable to form good pellets.

SUMMARY

Certain disclosed embodiments of the present application concern a process for converting fluffy, low density, fibrous biomass raw material into a form that facilitates subsequent torrefaction. Certain disclosed embodiments concern a process and system for pre-treating fibrous biomass prior to torrefaction comprising milling and densification to form the raw biomass into a dense, mechanically stable form.

Certain disclosed embodiments also concern a new product form produced as an intermediate in the torrefaction of fibrous biomass. The torrefaction process involves heating cellulosic material in the absence of oxygen. For certain embodiments, torrefaction is preferably conducted in a vertical bulk flow reactor by passing super heated steam through a moving bed of the fibrous biomass. But raw fibrous biomass, such as the EFB left over from palm oil extraction or simply the shredded stock from juniper, pine or other forest waste, is nearly impossible to process in a bulk flow torrefier due to its low bulk density and tendency of the fibrous mass to bridge and not flow through even a very large opening. Disclosed embodiments of the new intermediate product form may be produced as high density, short length “pucks” by a process comprising milling and compaction to form the raw biomass into a dense, mechanically stable form. This dense form allows for a more well-defined design for torrefaction by a torrefier reactor and associated facilities with the production of a more uniformly torrefied biomass. The stronger, denser treated feed stock has more predictable bulk flow properties, adequate gas permeability to allow for uniform torrefaction and reduced levels of fine particle fractions that can be lost during the torrefaction step. Post torrefication, the dense torrefied material is re-milled easily because of its more friable structure and is thus highly suitable for final compaction into hard, water resistant pellets or cubes. The final stable, water resistant and dense pellets or cubes are a highly desirable replacement fuel, such as fuel for powdered, coal-fired power plants. The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a photographic image of certain disclosed embodiments of a wafer-type puck product having a substantially higher density than feed stock used to produce the product, wherein the product is now ready for torrefaction (the red size arrow is approximately 25 millimeters).

FIG. 2 is a photographic image of a cuber machine modified to make disclosed product embodiments.

FIG. 3 is a photograph of EFB biomass comprising a loose fibrous mass having a bulk density of about 1-2 lbs/ft³.

FIG. 4 is a photograph of product made according to the present invention subsequent to torrefaction.

FIG. 5 is a schematic representation of a disclosed embodiment of a torrefaction reactor.

FIG. 6 is a schematic representation of a disclosed embodiment of a torrefaction reactor.

FIG. 7 is a flow diagram illustrating process steps and associated components of a torrefaction system.

DETAILED DESCRIPTION I. Definitions

The following explanations of terms and methods are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. The singular forms “a,” “an,” and “the” refer to one or more than one, unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise. As used herein, “comprises” means “includes.” Thus, “comprising A or B,” means “including A, B, or A and B,” without excluding additional elements. All references, including patents and patent applications cited herein, are incorporated by reference.

Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise indicated, implicitly or explicitly, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is expressly recited.

Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure pertains. Although methods and materials similar or equivalent to those described herein can be used to practice or test technology according to the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and are not intended to be limiting.

Biomass: Refers to various kinds of cellulose-containing materials including, by way of example and without limitation, forest waste, agricultural crops either grown specifically for energy production or as by-products of traditional agricultural activities, or cellulosic biomass from urban origin. Biomass may be obtained from forest thinning operations, as non-commercial “slash” from commercial logging operations or from purposeful agricultural operations that encourage fast growing cellulosic species, such as switch grass, corn stover, arundo donax. These biomass materials have a wide range of as-harvested physical sizes and shapes, as well as a highly variable amount of moisture, either as free water or bound water.

II. Method for Making and Product Made

A conventional agricultural “cuber” machine was modified to transform fibrous, low density cellulosic biomass into a mechanically stable form suitable for use as a feed stock to a torrefier process without requiring the addition of a “binder” or other such adjuvant. The conventional “cuber” comprises a rotating press wheel that forces low density, fibrous feed stock into a tapered die. This device was modified to permit the product to be produced in the form of thin wafer type “pucks” of relatively uniform size and with a density substantially higher than the feed stock. Modifications of the standard cuber and method for its use included altering the shape of the dies to form cubes, adding a controller to provide a controlled extrusion length for the thin puck shapes, controlling the temperature of the dies to assure the extrusions are mechanically strong, such as by using a temperature of from about 70° C. to about 150° C., and controlling the moisture content of the feed stock to a level of from about 3% to about 10% to provide a suitable mechanical strength of the extruded product. Lower moisture content typically is better so long as feedstock is properly compressed into a solid piece. If the moisture content is higher than 10%, cubes may crack and they also do not properly separate during the production process.

Certain disclosed embodiments of the product concern a compact “cube” or “thin puck” of raw cellulosic biomass having a density of 20 to 32 lb/ft³, which is 4 to at least 15 times the bulk density of the shredded raw biomass. The moisture content is 10% or less and more typically is below 10%, such as from 3 to 8%. The product has more than adequate mechanical strength for commercial applications. Mechanical strength can be measured by a drop test. For example, one drop test that has been used to test product made according to the present invention comprises dropping product onto a hard surface from a height of 3 feet. Products according to the present invention typically produce less than 10% breakage. These drop tests establish that the densified biomass has sufficient mechanical strength to withstand normal handling without break-down to its original fine particle size. With no added moisture or other additives to facilitate forming the desired shape, disclosed product embodiments may be converted with low energy utilization into a high energy density product, such as is described in U.S. Pat. No. 9,206,368, which is incorporated herein by reference.

The dense pucks of raw shredded biomass are produced, for example, using a modified Warren & Baerg Model 250W Cuber. Modifications to this exemplary known cuber included positioning a deflector bar 2 positioned radially outward from the end of the cuber's dies as shown in FIG. 2. The shredded biomass is initially milled in a hammer mill to a size of less than 10 mm. This shredded and milled biomass is then fed to the cuber. In the cuber operation, as the biomass is extruded through the dies, the extrudate contacts the deflector bar, which breaks the extrudate to form puck-shaped solids (FIG. 1). The densified biomass pucks are then ready for processing in a bulk flow torrefier, as described by U.S. Pat. No. 9,206,368 or similar process.

The products of the present invention can be produced having any desirable dimensions. The exemplary products illustrated by FIG. 1 have a substantially square, rectangular or parallelogram shape having dimensions corresponding to the dimensions of the die 2 in FIG. 2. The length of each extrudate is determined by the angle of the deflector plate 1 in FIG. 2. Certain disclosed embodiments have a length that is between about 5 mm to about 30 mm. As the cuber's press wheel rotates past the inlet side of the die, the raw biomass is compressed and forced into the die. Each die may be tapered along its length to provide back pressure, which increases the overall compressive force. With each rotation of the press wheel, a fresh increment of raw biomass is pushed into the die. Some longitudinal compression occurs such that the final extrudate exiting the die has substantial longitudinal strength. When the extrudate contacts the deflector plate 2, the extrudate breaks and the result is the relatively small “pucks” shown by FIG. 1. The model 250 Cuber used for this demonstration example comprised 66 dies arranged around the head of the unit.

III. Torrefaction

The densified biomass pucks made according to the present invention may be processed by a torrefier. One suitable torrefier and process for using the torrefier are described below. A person of ordinary skill in the art will appreciate, however, that other torrefaction apparatuses and processes also can be used to process densified biomass feed products made according to the present invention.

Certain embodiments of one suitable torrefaction reactor are represented by FIGS. 5 and 6. The illustrated reactor can be configured as a vertical disposed cylindrical or rectangular vessel (5) having an upper straight walled section (20), defining a reaction chamber, and a lower tapered bottom section (22) defining a cooling zone. One particular reactor is configured as a cylindrical vessel for ease of construction and management. The angle of the tapered section has an angle from the vertical of 25° or less, and advantageously is within the range of 10° to 25°, and more advantageously from 12° to 18°.

One consideration for a suitable torrefaction system is the ability to control the residency time of the material within the reactor to achieve satisfactory and uniform torrefaction of the processed material. Presently disclosed embodiments typically operate in a continuous mode, as opposed to batch mode. The reactor may be equipped with particulate discharge or discharge means comprising an opening, typically an ovoid or spherical opening, located at the base of the tapered bottom section. The particulate discharge or discharge means typically has a dimension of at least 200 mm or more in the shortest cross-section.

Torrefaction of biomass processed according to the present invention generally involves applying heat to induce conversion of the raw biomass to torrefied biomass. Heat may be provided by introducing hot gases into the torrefaction reactor. Accordingly, the reactor is provided with a heated gas input (19) positioned at the top of the tapered bottom section or bottom of the upper straight-walled section and comprises a device able to introduce the heated gas around the perimeter of the reactor. Typically such device includes one or more injectors, or a plenum having numerous orifice holes sized and spaced apart so as to assure even distribution of the hot torrefying gases while minimizing the system pressure drop. The region of the reactor between the hot gas inlet (19) and the gas outlet (27) is the torrefaction zone. The entry point of the hot gases into the torrefier reactor at a point (21) corresponds to where the stress on the biomass is higher than in the bulk of the reactor. That is, at a location where the cross sectional flow area of the mass flow reactor is becoming constrained, at the point where the straight sides of the reactor meet a conical shaped lower portion. The hot gases pass through the torrefaction zone contacting enroute any particulate biomass within the zone and exit from zone as torrefaction gases from the reactor, generally at the top of the reactor.

In operation, biomass processed according to the present invention enters the top of the mass flow reactor (25) and after descending the reactor in a mass flow mode, moving against the upward flow of hot gases, the torrefied biomass leaves the lower discharge point of the reactor (24). By suitable design of the sloped walls of the mass flow reactor, the entire mass remains in the torrefaction zone of the reactor for a uniform and controlled period of time. As noted above discharge of the torrefied biomass occurs via the particulate discharge means. To further facilitate control of the rate of discharge the system may be equipped with a discharge regulating device located externally to the particulate discharge means and wherein the device comprises a conveyor or airlock.

In a preferred embodiment, the system disclosed herein is further equipped with a temperature sensing means, or temperature sensor, able to determine the temperature of the particulate biomass within the reaction temperature. The temperature sensing means, or temperature sensor, is further in communication with the discharge regulating device and together function to control the rate of discharge of torrefied particulate biomass from the system. In this manner, the residency time of the particulate biomass within the torrefaction reactor is controlled by function of its temperature, thereby ensuring a correct and desired degree of torrefaction. And by using a reactor having mass flow characteristics the uniformity of the degree of torrefaction is consistent across the bulk mass of the material.

For successful torrefaction of biomass processed according to the present invention for use as a solid fuel, uniform and controlled torrefaction of biomass is desired. Incomplete torrefaction results in a product which will be problematic in grinder operations due to a higher modulus (flexibility and toughness). Over-torrefied material loses more of its energy as high fuel value compounds are driven off at long residence time or higher temperatures.

A functional block flow diagram of the overall process is shown in FIG. 7. Biomass is processed according to the present invention for use as feed material to the reactor. The feed material (1) is first converted to a specified size in a conventional grinder (3). The grinder reduces this size to a maximum of 13 mm×75 mm. Any conventional grinder may be used, such as a horizontal tub grinder commonly used in the forest products industry.

The processed biomass sized by the grinder is dried in a continuous direct air heated dryer (4). This dryer may be of the bulk flow type or any dryer suitably configured for this service. The dryer (4) delivers a product having a controlled residual moisture content of 25 wt % or less based on total weight of the biomass, and advantageously the residual moisture content is from about 12 wt % to about 25 wt %. The heated air for the dryer is a combination of hot combustion gases from an auxiliary heater (11) combined with cooled combustion gases (18) from the thermal oxidizer (9) associated with the torrefaction reactor (5). Fuel (13) combined with combustion air (12) in the auxiliary heater (11) provides the balance of thermal energy for operating the dryer (4). The dryer (4) delivers a product having a controlled residual moisture content of 25 wt % or less to the torrefier (5).

From the dryer (4) the biomass having 25% or less moisture content and with a size of from about 13 mm to about −75 mm in the longest dimension is fed to the bulk flow torrefier (5). Based on the particle size of the pre-sized biomass feed the vertical angle of all non-vertical surfaces (A, B) have been previously determined by a series of tests conducted on representative samples of the biomass. In the case of Eastern Oregon Juniper, for example, the maximum angle (A) in FIG. 5 would be 16°. The biomass enters the torrefier (5) at the top via a rotary air lock or similar atmosphere control device (25). Such rotary air lock or atmospheric control device is required to prevent ingress of oxygen into the reactor as torrefaction of biomass occurs at elevated temperatures in the substantial absence of oxygen. The torrefier is of the mass or bulk flow configuration (FIGS. 5, 6). That is, it has a cylindrical (FIG. 5) or rectangular (FIG. 6) body with a diameter or diagonal (D) and height (reaction zone) sized to provide the required residence time for the reaction. The volume of the reaction zone of the vessel allows the biomass to be heated to the torrefaction temperature of from 240° C. to 280° C. Controlling charging and discharging rates provides a residence time at the maximum temperature of from about 5 minutes to about 15 minutes. Exposure to temperatures greater than this promotes pyrolysis of the biomass and detracts from its calorific value as torrefied biomass. If the residency time is too short the raw biomass does not undergo full conversion to a torrefied biomass.

The hot gases enter the torrefier through air inlet(s) (19) located at the junction of the straight sides of the reactor (20) and the elongated cone shaped lower section (22). The location of the hot gas inlet is at a point where the stress on the mass charge is greatest. An internal flow splitter (23) having sloped sides with angles equal to the slope of the walls further increases the stress in the particulate solids mass. At that point there is the least tendency for the charge to become fluidized and this location promotes the greatest and most even distribution of the hot gases throughout the downward moving mass. The hot gases entering at a temperature of about 300° C. move upward through the downward moving mass. The decomposition of the biomass and removal of the last amount of moisture and torrefaction reaction gases occurs as the hot gases move upward. This method permits the maximum temperature of the torrefied biomass to be limited by modulating the temperature of the torrefying gases circulated through the bulk flow reactor.

The reaction gases exit the top of the torrefier (27) and flow through an external heat exchanger (10) where the gases are reheated by combustion gases from a thermal oxidizer (9). A portion of the reaction gases, which represents the residual moisture and the released decomposition gases (16), flow to the thermal oxidizer (9) where, combined with metered air (14) and if necessary auxiliary fuel (15), are combusted. After re-heating the circulating torrefier gases (17) in heat exchanger (10) the cooled combustion gases (18) flow to the dryer (4) to augment the heat required there.

Below the gas inlet(s) (19) of the torrefier (5), the torrefied biomass is cooled by contact with a jacketed section (26) of the torrefier (5). The coolant in this area may be any suitable coolant, such as water or other heat transfer fluid. The temperature of the coolant is maintained above the dew point of the hot gases in the torrefier, generally above about 80° C. The downward moving mass is cooled to below 150° C., which is its auto-ignition temperature in air.

The cooled torrefied biomass is discharged from the lower conical section of the torrefier via an opening (26) to a rotary air lock, or preferably a graduated pitch screw conveyor. The discharge opening (24) is an elongated slot whose smallest dimension was determined by a series of tests using the typical process biomass. The minimum dimension of the elongated discharge opening to provide for a bulk flow condition generally is about 200 mm. The torrefied biomass as discharged may still have a temperature significantly greater than the ambient air temperature; accordingly, it is desirable to manage this temperature by advantageously subjecting it to a cooling step to mitigate any risk of spontaneous combustion on exposure to ambient air.

This method permits the properties of the torrefied biomass to be controlled by modulating the rate of withdrawal of the torrefied biomass from the bulk flow torrefier. A useful discharge means is a screw conveyor in which the flights of the conveyor increase in the direction of the discharge flow in order to facilitate bulk flow from the reactor.

The cooled torrefied product from the reactor (5) is optionally milled (7) to a smaller size suitable for densification (8) to yield a final, torrefied, high density fuel (2).

IV. Example

The following example is provided to illustrate certain features of a particular working embodiment. A person of ordinary skill in the art will appreciate that the scope of the present invention is not limited to these particular features.

Raw EFB biomass shown in FIG. 3 is a loose fibrous mass having a bulk density of about 1-2 lb/ft³. Its low density and highly interlocking fibrous nature makes it very difficult if not impossible to process in a desirable bulk flow reactor.

Dried whole EFBs were hammer milled. Chopped EFB was cubed. A thin puck insert design was then developed, as well as a method of cooling the dies.

After processing through the modified cuber, the bulk density of the biomass material was increased to about 25 lb/ft³. (FIG. 1). After torrefaction, the material shown in FIG. 1 became a brownish-black mass (see FIG. 4) having a high energy density.

In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. 

We claim:
 1. A method, comprising: providing biomass; and processing the biomass to produce a mechanically stable puck suitable for use as a torrefaction feed stock without requiring a binder.
 2. The method according to claim 1, comprising processing the biomass using a cuber comprising a rotating press wheel that forces low density, fibrous feed stock into a straight or tapered die that forms the puck, wherein the puck has a relatively uniform size and a density substantially higher than the feed stock.
 3. The method according to claim 2 wherein the biomass has an initial density of 1-2 lb/ft³.
 4. The method according to claim 3 wherein the puck comprises raw cellulosic biomass having a density of from 4 to 15 times the bulk density of the shredded raw biomass.
 5. The method according to claim 4 wherein the puck has a density of from 20 to 32 lb/ft³.
 6. The method according to claim 2 wherein the cuber comprises: a controller to provide a controlled extrusion length for thin pucks produced by the process; and a temperature controller to control the temperature of the dies to a a temperature of from about 70° C. to about 150° C.
 7. The method according to claim 1 wherein the moisture content of the puck is less than 25%.
 8. The method according to claim 7 wherein the moisture content of the puck is from about 3% to about 10%.
 9. The method according to claim 4 wherein the density of the puck is from 20 to 32 lb/ft³ and the moisture content is from 3% to 8%.
 10. The method according to claim 1 wherein the puck has a substantially square, rectangular or parallelogram shape having dimensions corresponding to the dimensions of the die.
 11. The method according to claim 1, further comprising: using the puck as a feed stock for a torrefaction reactor; and torrefying the mechanically stable form.
 12. The method according to claim 11, comprising: converting the puck to a feed stock having a smaller size than the puck; optionally drying the feed stock using a continuous direct air heated dryer to provide a dried feed stock having a controlled residual moisture content of 25 wt % or less based on total weight of the biomass; feeding the dried feed stock to a torrefier to produce a torrefied product; cooling the torrefied product; and discharging cooled torrefied product from the torrefier.
 13. The method according to claim 12, further comprising: milling the cooled torrefied product to a smaller size suitable for densification; and densifying milled, torrefied product to produce a torrefied, high density fuel.
 14. A method, comprising: providing a product made according to claim 1; and using the product as a torrefaction feed stock.
 15. A method, comprising: providing a product made according to claim 11; and using the product as a fuel.
 16. A product, produced according to the method of claim
 1. 17. A torrefied product, produced according to the method of claim
 11. 